Oral drug administration remains the route of choice for the majority of clinical applications. Modified release (MR) dosage forms that are administered once or twice daily offer advantages over their immediate release (IR) counterparts because they reduce the magnitude of peaks and troughs of drug plasma concentration, provide longer dosing intervals, sustained analgesic effect, and increased patient compliance. These modified release formulations may be referred to as controlled release (CR), sustained release (SR) and/or extended release (ER) etc. For certain types of patients, such as those suffering from pain, these MR products may permit the patient to sleep through the night without having to wake up during the night to take the next dose. Thus, it can significantly increase the quality of life for such patients. Both IR and MR products for pain are widely available in the market. Examples of IR products include those containing NSAIDs, opioids, profens, COX II inhibitors and aspirin (Tylenol, Advil, Celebrex, Vioxx, Aleve, Voltaren). Examples of MR products include those containing NSAIDs and opioids (Tylenol SR, Oxycontin).
Researchers have also combined various classes of pain drugs to provide better analgesia to patients. For example, a combination of acetaminophen-hydrocodone bitartrate is commercially available as Vicodin, and acetaminophen-oxycodone hydrochloride is commercially available as Percocet. In randomized controlled trials, it was shown that the combination product Percocet was statistically superior to MR hydrocodone in various outcome measures of pain relief. Other combination products such as acetaminophen-tramadol are either available or described in the literature. It is postulated that the combination of two analgesic drugs with complementary mechanisms of action results in enhanced analgesia due to an additive effect, an “opioid-sparing” effect, and an improved side effect and safety profile. The improved safety profile results from the use of reduced doses of two analgesics with different side-effects rather than an equieffective dose of a single agent.
Acetaminophen is absorbed from the small intestine and primarily metabolized by conjugation, like glucuronidation and sulfation, in the liver to nontoxic, water-soluble compounds that are eliminated in the urine. When the maximum daily dose is exceeded over a prolonged period, metabolism by conjugation becomes saturated, and excess acetaminophen is oxidatively metabolized by cytochrome P450 (CYP) enzymes (e.g., CYP2E1, 1A2, 2A6, 3A4) to a reactive metabolite, N-acetyl-p-benzoquinone-imine (NAPQI). NAPQI is a reactive free radical with an extremely short half-life that is rapidly inactivated by conjugation with glutathione, which is acting as a sulfhydryl donor. Once the pool of available glutathione is exhausted, the cysteines of cellular proteins become sulfhydryl donors to NAPQI, binding covalently and initiating a cascade of oxidative and cellular damage, resulting in necrosis and, ultimately, liver failure. Thus, avoiding excessive NAPQI formation is an important strategy when using acetaminophen, although to date acetaminophen-sparing has not been an approach any manufacturers have chosen to take. However, due to the prevalence of acetaminophen in many over-the-counter products, it is prudent to consider acetaminophen-sparing precautions when considering combination therapy lasting more than a few days to avoid an inadvertent reduction in glutathione stores.
Thus, various options for pain management are available that are both IR and MR, and contain either a single drug or a combination of analgesics. While these combination products provide the benefits associated with combining two analgesics as described above, both IR and MR, in itself, have a significant disadvantage. IR combination products lack the advantages of MR products described previously. MR combination products lack a significant benefit associated with IR products—rapid onset of analgesia—that is extremely desirable for pain management. Because MR products retard the rate of drug release to sustain the drug effect over prolonged period, release of drug is slow resulting in significant time before effective analgesic drug concentration is attained in the bloodstream. There exists a clinical need for pain management that combines the desirable features of IR and MR in combination pain products.